The present invention relates to a gas sensor installed in an exhaust system of an automotive internal combustion engine to detect an oxygen concentration in the exhaust gas, or an air-fuel ratio, or the like.
The present invention relates to a gas sensing element used for controlling an air-fuel ratio of an internal combustion engine and a method for manufacturing the gas sensing element.
In general, to control the air-fuel ratio, a gas sensor is installed in an exhaust system of an automotive internal combustion engine.
The gas sensor comprises a gas sensing element provided at its front end for detecting an oxygen concentration. The gas sensing element comprises a solid electrolytic sintered body having oxygen ion conductance, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The measured gas side electrode is covered by a porous electrode protective layer.
In many cases, the electrode protective layer is a ceramic coating layer, or a double layer consisting of a ceramic coating layer and a xcex3-A1203 layer provided on this ceramic coating layer.
According to this type of gas sensing elements, a measured gas reaches a measured gas side electrode through the ceramic coating layer or the double layer of the ceramic coating layer and the xcex3-A1203 layer. The gas sensing element produces a sensor output.
Recent radically changing circumstances, such as enhancement of emission control laws and regulations as well as requirement to high power internal combustion engines, forces automotive manufacturers to develop automotive engines capable of precisely controlling the combustion.
To realize this, it is essentially important to provide excellent gas sensors having sensing properties stable under severe operating conditions and durable for a long-term use.
FIG. 5 shows a characteristic curve representing a relationship between air-fuel ratio and voltage, as important sensor output characteristics of a gas sensing element used for combustion control of an internal combustion engine. In FIG. 5, point xcex is referred to as a specific air-fuel ratio where the voltage causes steep changes. In FIG. 5, a reference voltage is a criteria used for judging whether a fuel injection amount should be increased or decreased in the combustion control of an internal combustion engine. In general, the reference voltage is set to 0.45V.
More specifically, when a sensor output is larger than the reference voltage, the fuel injection amount is reduced to form an air/fuel mixture whose air-fuel ratio is shifted to a lean side. On the contrary, when a sensor output is less than the reference voltage, the fuel injection amount is increased to form a relatively rich air/fuel mixture. Through such a feedback control, the air-fuel ratio of the controlled engine can be always kept in a window of a ternary catalyst.
Accordingly, to precisely perform the air-fuel ratio control, it is essentially important to stabilize the point xcex (hereinafter, referred to as control xcex).
In other words, the control xcex should be stable during a long-term use of a gas sensing element and should be constant regardless of any environmental change of the gas sensing element.
When a gas sensing element is installed in an exhaust system of an internal combustion engine, a sensor output is produced in the following manner.
First, an exhaust gas containing unburnt components reaches a measured gas side electrode. Then, an equilibrium oxygen concentration is obtained through a catalytic reaction caused on the measured gas side electrode. The sensor output is produced as a signal representing a difference between the equilibrium oxygen concentration thus obtained and an oxygen concentration in the air serving as a reference gas.
Accordingly, it becomes possible to increase the measuring accuracy of a gas sensing element when an electrode having excellent activity is used as a measured gas side electrode of a gas sensing element.
The following is a method for activating a measured gas side electrode disclosed in Unexamined Japanese patent publication No. 10-104194.
First, a measured gas side electrode is formed on a surface of a solid electrolytic body by baking in the air at the temperature range from 1,000xc2x0 C. to 1,400xc2x0 C. Then, a heat treatment is applied to the measured gas side electrode thus formed in an atmosphere containing H2.
Subsequently, a heat treatment in an inert atmosphere and a heat treatment in a non-oxidative atmosphere including moisture vapor are applied to the measured gas side electrode.
By combining these treatments, the catalytic activity of the measured gas side electrode can be enhanced.
However, according to the above-described conventional method, it was difficult to provide a gas sensing element having a measured gas side electrode which can assure a sufficiently stable control xcex even in a severe high-temperature environment or in a poisonous environment containing Si compounds.
In view of the foregoing problems of the prior art, the present invention has an object to provide a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
To accomplish the above and other related objects, the present invention provides a first gas sensing element comprising a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas, wherein a crystal face strength ratio of the measured gas side electrode according to X-ray diffraction is 0.7xe2x89xa6{I(200)/I(111)} or 0.6xe2x89xa6{I(220)/I(111)}.
The first gas sensing element of the present invention is characterized in that the measured gas side electrode has a crystal face strength ratio according to X-ray diffraction satisfying the above-described conditions.
If I(200)/I(111) is less than 0.7, a ratio of an active surface to an entire electrode surface will reduce to 0.5 or less and it will be difficult to assure satisfactory catalytic activity and stability for smoothly promoting an equilibrating reaction of exhaust gas.
If I(220)/I(111) is less than 0.6, it will be difficult to assure satisfactory catalytic activity and stability.
To obtain crystal grains and an electrode film which are stable in energy level and easily fabricable, a preferable upper limit of the crystal face strength ratio is 1.0 in view of the fact that the total area of active faces (200) and (220) can be maximized and because according to this condition the crystal face orientation can satisfy the requirement that the solid electrolytic body causes no alteration.
Next, functions and effects of the present invention will be explained hereinafter.
Inventors of this invention enthusiastically conducted research and development for stabilizing the activity of a measured gas side electrode, i.e., stabilization of control xcex. And, as a result of the research and development, the inventors have found the fact that a crystal face of the measured gas side electrode greatly contributes to activation and stability of a gas sensing element.
The measured gas side electrode is made of an electrode material containing noble metals which possess catalytic properties and usually have a face-centered cubic structure.
In a crystal lattice of such metals, specific crystal faces, i.e., faces (100) and (110), have a lower surface density of atoms compared with other face (111) dominant in this crystal lattice.
Due to lower surface densities, these faces (100) and (110) promote adsorption of various exhaust components. Thus, these crystal faces can smoothly adsorb unburnt exhaust components and residual oxygen when the measured gas is exhaust gas. The equilibrating reaction smoothly advances.
According to the measured gas side electrode satisfying the above-described requirements, the faces (100) and (110) are orientated on the surface of the measured gas side electrode.
In the X-ray diffraction of a noble metal having a face-centered cubic structure, both faces (100) and (110) appear as faces (200) and (220) respectively. Hence, the strength ratio of this invention is expressed by using faces (200) and (220). Namely, strengths of faces (100) and (110) can be replaced by those of faces (200) and (220). The same result is obtained.
As described above, according to the gas sensing element of this invention, the entire surface of the measured gas side electrode possesses higher activity. Even when the measured gas side electrode is exposed to a severe high-temperature environment or in a poisoning environment containing Si components, the equilibrating reaction of exhaust gas can advance smoothly on the electrode surface. In this respect, the measured gas side electrode of this invention possesses excellent catalytic properties. Thus, it becomes possible to provides a sensor whose output is stable even after a long-term use in the high-temperature environment and robust against Si poisoning.
Accordingly, the gas sensing element of the present invention can maintain a stable control xcex for a long time.
As apparent from the foregoing description, the present invention can provide a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
Furthermore, the present invention provides a second gas sensing element comprising a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas, wherein a crystal face strength ratio of the measured gas side electrode according to X-ray diffraction is 1.3xe2x89xa6{I(200)+I(220)}/I(111).
The measured gas side electrode of this gas sensing element has a crystal face strength ratio according to X-ray diffraction satisfying the above-described conditions.
If the crystal face strength ratio is less than 1.3, activity of the measured gas side electrode will be soon worsened in severe operating conditions, such as a high-temperature environment and a Si poisoning environment. The control xcex will vary widely depending on the operating conditions. Thus, the gas concentration cannot be measured accurately.
To obtain crystal grains and an electrode film which are stable in energy level and fabricable, a preferable upper limit of {I(200)+I(220)}/I(111) is 2.0 in view of the fact that the total area of faces (200) and (220) can be maximized and because this condition satisfies crystal face orientation requirement that the solid electrolytic body causes no alteration.
As described above, the measured gas side electrode is made of an electrode material containing noble metals which possess catalytic properties and usually have a face-centered cubic structure.
In a crystal lattice of such metals, both faces (100) and (110) have a lower surface density of atoms compared with other face (111) dominant in this crystal lattice.
Accordingly, these faces (100) and (110) promote adsorption of various exhaust components and act as active faces smoothly advancing the equilibrating reaction.
According to the measured gas side electrode satisfying the requirement 1.3xe2x89xa6{I(200)+I(220)}/I(111), the above-described active faces are orientated on the surface of the measured gas side electrode. Thus, it becomes possible to provides a sensor whose output is stable even after a long-term use in the high-temperature environment and robust against Si poisoning. Accordingly, the gas sensing element of the present invention can maintain a stable control xcex for a long time.
As apparent from the foregoing description, the present invention can provide a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
Furthermore, it is preferable that {I(200)+I(220)}/I(111) is equal to or larger than 1.5.
With this arrangement, it becomes possible to easily form an electrode having a uniform film thickness and possessing stable and excellent catalytic activity without causing alteration of a solid electrolytic body.
The strength ratio of the above-described crystal faces can be obtained by measuring the X-ray diffraction strength of a surface of the measured gas side electrode (to be exposed to a measured gas) according to the X-ray diffraction method, for example by using a position sensitive proportional counter (PSPC) type microdiffractometer, manufactured by Rigaku Corporation. Alternatively, the crystal face strength ratio can be obtained according to another X-ray diffraction method using a similar diffractometer.
The following is an example of practical measurement.
According to the X-ray diffraction method using the above-described measuring apparatus, a thin X-ray with a diameter in the range from 200 xcexcm to 300 xcexcm with a power of 40 kV and 80 mA is irradiated onto a surface of the measured gas side electrode of each tested element piece (whose size is equal to or less than 5 mm) fixed by a sample holder.
Then, the strength ratio of crystal faces is calculated based on data collected simultaneously by a PSPC program (capable of performing both measurement and data processing) manufactured by Rigaku Corporation, while X-ray diffraction angle (2xcex8) varies in the range from 20xc2x0 to 80xc2x0. This measurement is performed in the air at a room temperature. The measured data is subjected to correction based on a Si powder standard sample.
As described above, in the X-ray diffraction of a noble metal having a face-centered cubic structure, both faces (100) and (110) appear as faces (200) and (220) respectively. Hence, strengths of faces (100) and (110) can be replaced by those of faces (200) and (220) respectively.
When the noble metal having a face-centered cubic structure is random and does not have specific orientation, the strength ratio {I(200)+I(220)}/I(111) becomes 0.84.
Furthermore, the present invention provides a first method of manufacturing a gas sensing element comprising a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The first manufacturing method comprises a step of providing a fine-grain nucleus (or a particulate core) of noble metal on an electrode forming portion of the solid electrolytic body, a step of applying a reducing heat treatment to the fine-grain nucleus, a step of forming a unbaked film electrode on the fine-grain nucleus, and a step of baking the film electrode in a reducing atmosphere to form the measured gas side electrode.
By utilizing the first manufacturing method of this invention, it becomes possible to form a measured gas side electrode from a fine-grain nucleus having active faces, such as faces (100) and (110), having lower surface densities of atoms.
According to the first manufacturing method, a fine-grain nucleus of noble metal is provided on an electrode forming portion (i.e., a portion where a measured gas side electrode is to be provided) of a solid electrolytic body. Then, a reducing heat treatment is applied to the fine-grain nucleus.
Grains of noble metal have higher surface energy. Applying a reducing heat treatment to a fine-grain nucleus induces chemical adsorption of reducing gas molecules in addition to thermal action.
As a result, active crystal faces having higher energy level, i.e., faces (100) and (110) having lower atomic surface densities, are formed on the fine-grain nucleus.
In a reducing atmosphere, the above-described active faces chiefly adsorb the molecules having strong reducing properties and higher activity, such as hydrogen molecules and carbon monoxide molecules. Growth of these active faces becomes slow compared with that of other crystal face. As a result, the percentage of a crystal face (111) having a higher growth rate reduces and the active faces having slow growth rates remain in a wide region.
Therefore, in a case where a unbaked electrode film is formed on the fine-grain nucleus having active faces and this electrode film is baked in a reducing atmosphere, the orientation of the active faces can be maintained during the growth of the fine-grain nucleus. Thus, the electrode can be formed.
Accordingly, it becomes possible to obtain a gas sensing element having a measured gas side electrode on the surface of which active faces are orientated. The measured gas side electrode, when the active faces are dominant on the surface thereof, can produce a stable output even after a long-term use in the above-described high-temperature environment and can possess preferable durability against Si poisoning. Accordingly, the gas sensing element of the present invention can maintain a stable control xcex for a long time.
As apparent from the foregoing description, the present invention can provide a method for manufacturing a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
The above-described electrode film can be formed by utilizing chemical (i.e., electroless) plating, sputtering, vaporization etc.
A material can be used as the fine-grain nucleus when it includes at least one noble metal having a face-centered cubic structure such as Pt, Rh, Ir, Pd, Au etc.
The first manufacturing method of this invention can be put into practice in the following manner. First, a Pt nucleus is formed on a surface of a solid electrolytic body by reducing a chloroplatinic acid solution. Then, the Pt nucleus is subjected to the reducing thermal processing in a reducing atmosphere at a high temperature. Then, an electroless plating is applied to the Pt nucleus, thereby forming a plating layer on the Pt nucleus. Thereafter, the plating layer is thermally treated in a reducing atmosphere to form a measured gas side electrode.
It is preferable that the reducing heat treatment for the fine-grain nucleus is performed in the temperature range from 600xc2x0 C. to 800xc2x0 C.
If the temperature is less than 600xc2x0 C., the effect of orientating the active faces on the fine-grain nucleus will be reduced. If the temperature is higher than 800xc2x0 C., mutual bonding and grain growth will be caused between fine-grain nucleuses. No fine-grain nucleus may be formed on the electrode forming portion of a solid electrolytic body. The electrode film cannot be formed in an intended manner.
Furthermore, H2-N2 series atmosphere can be preferably used for the reducing heat treatment applied to the fine-grain nucleus according to this invention.
Hydrogen atoms are selectively adsorbed on faces (100) and (110) of the crystal lattice constituting the fine-grain nucleus. This promotes the formation of active faces.
Furthermore, it is preferable that the concentration of H2 contained in the reducing heat treatment atmosphere is equal to or larger than 5 vol %.
If the H2 concentration is less than 5 vol %, the active faces may not be formed due to lack of H2 in the reducing heat treatment atmosphere.
Furthermore, the present invention provides a second method of manufacturing a gas sensing element comprising a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The second manufacturing method comprises a step of providing a fine-grain nucleus of noble metal on an electrode forming portion of the solid electrolytic body, a step of irradiating a laser beam to the fine-grain nucleus, a step of forming a unbaked electrode film on the fine-grain nucleus, and a step of baking the electrode film in a reducing atmosphere to form the measured gas side electrode.
Like the above-described reducing heat treatment, performing the laser irradiating processing is effective to activate and rearrange the surficial atoms of the fine-grain nucleus. Accordingly, it becomes possible to obtain a gas sensing element having a measured gas side electrode on the surface of which active faces are orientated. The measured gas side electrode, when the active faces are dominant on the surface thereof, can produce a stable output even after a long-term use in the above-described high-temperature environment and can possess preferable durability against Si poisoning. Accordingly, the gas sensing element of the present invention can maintain a stable control xcex for a long time.
As apparent from the foregoing description, the present invention can provide a method for manufacturing a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
The second manufacturing method of this invention can be put into practice in the following manner. First, a Pt nucleus is formed by reducing a chloroplatinic acid solution and is subjected to the laser irradiation processing. Thereafter, an electroless plating is applied to the Pt nucleus, thereby forming a plating layer comprising a Pt grain polycrystal having excellent orientation.
Then, the Pt plating later is baked in an inert atmosphere to obtain a measured gas side electrode having excellent orientation.
Regarding the laser irradiation, a preferable laser power is in the range from 10 mW to 50 mW and a preferable irradiation time is in the range from 1 minute to 30 minutes.
When the laser irradiation is performed within the above irradiation time, increasing the laser power up to 50 mW will give a significant damage to a solid electrolytic body and accordingly will cause local alterations on the solid electrolytic body (i.e., black spots).
Furthermore, if the irradiation time exceeds 30 minutes, the fine-grain nucleus will agglutinate or partly evaporate. The electrode film thus formed will not have a uniform film thickness.
If the laser power is less than 10 mW, or if the irradiation time is shorter than 1 minute, no orientation will be caused on the fine-grain nucleus. The effects of the present invention will not be obtained.
Furthermore, the present invention provides a third method of manufacturing a gas sensing element comprising a solid electrolytic body, a reference gas side electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas side electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. The third manufacturing method comprises a step of preparing a paste of fine-grain nucleus of noble metal applied crystal face orientation processing beforehand, a step of forming a unbaked electrode film by coating the paste on an electrode forming portion of the solid electrolytic body, and a step of baking the electrode film in a reducing atmosphere to form the measured gas side electrode.
Orientating crystal faces of the fine-grain nucleus of noble metal makes it possible to form the active faces, i.e., faces (100) and (110), having higher energy levels.
According to the present invention, a paste is prepared so as to include the fine-grain nucleus of noble metal applied the crystal face orientation processing beforehand. Then, an electrode film is formed by using this paste. Then, a reducing heat treatment is applied. The fine-grain nucleus grows while maintaining the orientation of the active faces.
Accordingly, as the electrode film is formed and baked while maintaining the orientation adequately, it becomes possible to obtain a gas sensing element having a measured gas side electrode on the surface of which active faces are orientated.
As described above, the measured gas side electrode can produce a stable output even after a long-term use in the above-described high-temperature environment and can possess preferable durability against Si poisoning. Accordingly, the gas sensing element of the present invention can maintain a stable control xcex for a long time.
As apparent from the foregoing description, the present invention can provide a method for manufacturing a gas sensing element capable of demonstrating excellent performances in the heat resistivity as well as in the Si poisoning durability.
The third manufacturing method of this invention can be put into practice in the following manner.
First, a chloroplatinic acid is thermally decomposed in an inert atmosphere at the temperature range from 1,000xc2x0 C. to 1,100xc2x0 C. to obtain a Pt nucleus with active faces having excellent orientation. The Pt nucleus thus obtained is mixed with a binder and the like to form a paste. Then, the paste is baked in an inert gas atmosphere to obtain a measured gas side electrode whose active faces have excellent orientation.
Furthermore, in any of the above-described manufacturing methods of the present invention, a preferable baking temperature in a reducing atmosphere is in the range from 1,000xc2x0 C. to 1,100xc2x0 C.
The measured gas side electrode baked in this temperature range has numerous micro pores which can enhance gas diffusing performance. Furthermore, it becomes possible to improve the response of sensor output.
If the baking temperature exceeds 1,100xc2x0 C., alteration of a solid electrolytic body will be caused.
Application of the present invention is not limited to a cup-shaped gas sensing element (refer to FIGS. 1 and 2). Therefore, the present invention can be applied to another type of gas sensing elements, such as a multilayered planar gas sensing element consisting of a planar solid electrolytic body, a planar measured gas side electrode or the like stacked successively.
Furthermore, application of the present invention is not limited to an oxygen concentration cell type element. Therefore, the present invention can be applied to another type of gas sensing elements, such as a limit-current type element, a lean sensor type element, and an air-fuel ratio sensing element.
Furthermore, the gas sensor of the present invention can be used as a NOx sensor, a HC sensor, or a CO sensor.